A corruption scandal rocking the Czech government involves power and money but also, at its heart, questions about the nature of the relationship between the prime minister and a trusted female aide.

Jana Nagyova, who runs Prime Minister Petr Necas's office, is in custody, accused, among other things, of illegally ordering military intelligence agents to conduct surveillance on three unnamed individuals.

Nagyova's lawyer Eduard Bruna said his client was denying some parts of the prosecution charges, and on others was arguing that she had acted in good faith. He said Necas had been unaware of her actions.

Tomas Sokol, the lawyer representing the former head of military intelligence Ondrej Palenik, who is another defendant in the case, told Reuters one of the targets of the alleged surveillance was the prime minister's wife, Radka.

Necas this week announced that he and his wife, who was his college sweetheart and with whom he has four children, were jointly filing for divorce.

The details of what Nagyova is alleged to have done, and why, have not been officially released.

But asked at a news conference on Friday why she carried out the alleged illegal acts, state attorney Ivo Istvan, who is leading the investigation, said: "You would be surprised with (her) motive, as we were surprised. It was purely private."

Nagyova is among eight people charged this week with corruption and abuse of power. A court in the eastern city of Ostrava ruled on Saturday that she should remain in custody.

Speaking outside the court, her lawyer, Bruna, told reporters: "The (surveillance) was not done for the reason of damaging someone or harming someone but rather, due to being cautious, she reached the conclusion that it is necessary to find out some things."

The prosecutors' allegation that Nagyova instructed military intelligence to carry out surveillance, which would represent an abuse of her position, is at the centre of their case.

POLICE RAIDS

She was among people detained in a series of raids in the past week by hundreds of police officers from the organised crime unit, some of them in balaclavas, on government offices, banks and private homes.

The operation amounted to the biggest anti-graft sweep in 20 years in the Czech republic, a country where residents believe official corruption is pervasive, but high-profile politicians are rarely convicted.

The scandal has left the prime minister's political survival hanging in the balance.

Necas denies that he did anything dishonest and has said he will not resign. But a junior partner in the governing coalition has said it will consider over the weekend whether to withdraw its support, a step that would bring down his government.

If prosecutors can demonstrate that Nagyova acted because of her relationship with Necas, it could make it much harder for him to argue to voters and coalition partners that any alleged illegal activity had nothing to do with him.

Nagyova, born in 1965, has worked with Necas since 2006. Two government insiders said Nagyova was fiercely protective of the prime minister and had become a powerful gatekeeper for anyone who needed access to him.

"She was irrational, had jealous control of access to Necas," a senior government source, who knows Nagyova and requested anonymity because of the sensitivity of the issue, told Reuters.

Prosecutors have not suggested that Necas and Nagyova, who is not married, were romantically involved. Necas has never said he had anything but a purely professional relationship with Nagyova.

Asked at a late-night news conference on Friday about media reports suggesting that Nagyova put pressure on Necas to divorce his wife, the prime minister said: "I consider these things to be so bizarre that they do not deserve my comment."

Nagyova's lawyer was also asked about the reports, and said: "I asked her about that and she told me that it is absolute nonsense."